1. Field of the Invention
The present invention relates to a fixing apparatus, which is provided in an image forming apparatus such as a copier and a printer, and fixes a developer image on a paper sheet.
2. Description of the Related Art
An image forming apparatus such as an electric copier using digital technology has a fixing apparatus for fixing a heated and fused developer image to a paper sheet by applying a pressure.
A fixing apparatus has a heating member which fuses a developer such as toner, and a pressing member which supplies a predetermined pressure to the heating member. A predetermined contact width (nip width) is formed in the contacting area (nip) between the heating member and the pressing member. A developer image fused by the heat from the heating member is fixed to a paper sheet passing through the nip by the pressure from the pressing member.
In a method using a halogen lamp as a heat source of a heating member of a fixing apparatus, a pair of rollers is provided to be capable of supplying a predetermined pressure to a member to be fixed and toner, and a halogen lamp is provided in at least one of the rollers. This configuration has been widely used.
Contrarily, in a method using induction heating, it is well known that a heat-resistant film material is formed like an endless belt or cylindrical (roller), and brought it into contact with a material to be fixed.
There is another example (Jpn. Pat. Appln. KOKAI Publication No. 2002-49261). Silicon rubber or foamed silicon rubber is used as a low-heat conductive material in a fixing apparatus, which uses a fixing roller comprising a cylindrical rigid body, a layer made of a low-heat conductive material and provided outside the rigid body, and a conductive body layer and a mold release layer provided outside the low-heat conductive material layer, and having an induction heating source for heating the conductive body layer placed opposite to the outside of the mold release layer.
In this fixing apparatus, a conductive body layer with small heat capacity is provided outside a layer made of a low-heat conductive material, and quick heating is possible by using a roller body. However, the layer made of a low-heat conductive material is a silicon rubber or foamed silicon rubber layer with a foaming rate of 250%, and when it is used in a color fixing apparatus requiring a thick toner layer, a character image may be collapsed.
In an image heating unit in which a thin conductive layer, an insulation layer and a support layer are stacked closely contacted from the front surface toward the inside and heated externally by excitation, there is an example in which one of the surface of the insulation layer contacting the thin conductive layer and the surface of the thin conductive layer contacting the insulation layer, or the both are made rough (Jpn. Pat. Appln. KOKAI Publication No. 2001-5315). In this image heating unit, the curling direction of a paper sheet ejected at the exit of nip is the toner side on a recording material. This may cause winding of the recording material around the heating roller, or hot offset.
There is another example of a heating unit (Jpn. Pat. Appln. KOKAI Publication No. 8-76620). Conductive particles or whiskers with high magnetic permeability are scattered in an elastic layer or a front layer of conductive heating material, and these layers are functioned as an elastic layer or a conductive heating layer. In this heating unit, an adhesive is deteriorated and a surface is wrinkled, or a conductive body layer may be peeled off, because a conductive layer is formed by bonding with an adhesive.
There is still another example of a heating unit (Jpn. Pat. Appln. KOKAI Publication No. 8-129313). A heating rotary body including an elastic body layer and having a metal sleeve of 10-150 μm thick outside the elastic body layer is heated externally by a heating member. In this heating unit, quick heating is possible, but the conduction of heat to a support part contacting a core metal and a support part at both ends is large, and a temperature at the ends of a roller is increased when a small size paper sheet is passed.
In recent years, to decrease power consumption, quick heating is enabled by using a heating member with a thin conductive layer of small heat capacity provided outside a layer made of a low-heat conductive material. A foamed material with a small hardness is used for a low-heat conductive material, and the hardness becomes difference from a conductive body layer made of a thin film of metal or the like. Because of the hardness difference, the foamed material used as a low-heat conductive material may be consumed or the foam may be broken, when a pressure is applied from a pressing member or when a stress such as thermal expansion and thermal shrinkage is applied. If a broken foam is increased and cavities of different size are formed, the outside diameter of a conductive layer is not kept constant in the length direction of a heating roller.
In case where a conductive layer and foamed material are bonded by applying a heat-resistant adhesive to all over the contacting surface, a foamed rubber layer which receives the heat directly from a metal layer may be expanded suddenly by the heat. Further, a metal layer may restrict thermal expansion of a foamed rubber layer by the heated metal layer, or thermal shrinkage of a foamed rubber layer on heat radiation caused by reduction of the volume of air included in the foamed rubber layer.
Thus, a foamed rubber layer is expanded toward a metal layer by thermal shrinkage, and brake of a foamed rubber layer advances further, and the life (service life) is shortened. Further, as thermal expansion or shrinkage of a foamed rubber layer occurs, a metal layer is deformed, a required nip width is not ensured at a position where a heating member contacts a pressing member, and a good fixed image cannot not be obtained.
If a metal layer and a foamed rubber layer are not bonded with an adhesive, or a metal layer is provided as a sleeve (belt), a metal layer may meander, shift from a foamed rubber layer and break as a heating roller rotates, reducing the life of a metal layer. Moreover, a complicated mechanism is required to support a belt-like metal layer. When a metal belt is driven by the pressure from a pressing mechanism, the belt may be slipped by oil roller or the like, and may not be rotated.